Active photonic integrated circuits (PICs) in the visible spectrum are essential for on-chip applications, requiring low-loss waveguides with broad transparency and efficient, low-power phase modulation. Here, we demonstrate a compact, ultra-low-power phase modulator based on a silicon nitride (Si$_3$N$_4$) waveguide integrated with thin-film lead zirconate titanate (PZT) that actuates a bridge-type MEMS. The suspended actuator exploits PZT's strong piezoelectric effect to induce mechanically driven phase shifts, enabling efficient modulation in a Mach--Zehnder interferometer. For 3~mm and 5~mm modulators, phase shifts of $1.45π$ and $2.5π$ are achieved at 10~V, corresponding to a scalability metric ($V_π\cdot L$) of 2.25~V$\cdot$cm at 635~nm. This represents an order-of-magnitude improvement in scalability over stress-optic PZT modulators. The devices also exhibit ultralow power consumption ($\sim 12\,\mathrm{nW}$), $\sim 5\,\mathrm{ms}$ rise time, and optical loss $< 0.75\,\mathrm{dB/cm}$. Furthermore, we demonstrate on-chip beam shaping.
Photonic integrated circuit (PIC) technology is having a transformational effect on conventional free-space optics by integrating multiple optical components with diverse functionalities into a single compact chip. 1 Early PICs were predominantly passive, enabling static optical routing, splitting and filtering of light. More recently, tunable (active) and reconfigurable PICs have emerged, providing dynamic control of guided optical modes for modulation, switching and amplification. Such active control is essential for a wide range of on-chip photonics applications, including optical phased array 2 , photonic neural networks, 3 programmable PICs, 4 biomedical sensing, 5 bio-imaging, 6 quantum technologies 7 and light detection and ranging (LiDAR). 8 At the core of active PICs lie tunable elements that operate through mechanisms such as electro-optic modulation, thermal tuning, microelectromechanical systems (MEMS) actuation and stress-optic effects. 9 Tunable elements with these mechanisms enable key active photonics components, including phase and amplitude modulators, optical switches and tunable couplers. Active PICs are well established in the infrared (IR) regime, primarily using silicon (Si) via carrier injection or depletion, and indium phosphide (InP) through the electro-optic effect. 10,11 However, both Si and InP are limited to IR operation due to their material transmission windows. Lithium niobate (LiNbO3) has recently emerged as a leading material for broadband active PICs, offering a strong intrinsic electro-optic response and a wide transparency window extending from the visible to IR. 12 In contrast, silicon nitride (Si₃N₄) remains the most widely used platform for visible-spectrum PICs due to its mature fabrication process, low propagation loss, broad transparency (visible to IR), high power-handling capacity, and CMOS compatibility. However, unlike LiNbO₃, Si₃N₄ is inherently passive with poor thermo-optic, electro-optic and piezoelectric coefficients. To introduce active functionality, Si3N4-based PICs must therefore incorporate additional thin film materials with suitable tunable properties.
The focus of this work is development of novel active PIC using the Si₃N₄ platform for visible spectrum. Therefore, we summarize different phase modulator techniques explored so far for Si3N4-based PICs operating at visible wavelengths. These can be broadly classified as thermo-optic modulation, Pockels electro-optic (EO) modulation, stress-optic modulation and MEMS actuator-based modulators.
The most-widely reported integrated phase shifters for Si₃N₄ operating in the visible are based on the thermooptic effect. Thermo-optic modulation, which changes the refractive index through localized heating, has been implemented in Si₃N₄ PICs by integrating thin films such silicon dioxide, polymers and non-volatile phase change materials 13 . Improvements in the efficiency of this technique have focused on (1) enhancing thermal isolation using suspended structures: under cuts and trenches 14 , and (2) using advanced polymers with high thermo-optic properties 15 . While these designs show reduced power consumption and footprint of the active element, they often introduce trade-offs in optical loss, bandwidth, and fabrication complexity. The state-of-the art Si3N4 thermo-optic phase shifter 13 has a power consumption of around 1.2mW at 561 nm with an active element length of 1.5 mm and insertion loss of 5.5dB/cm. Electro-optic (Pockels) modulators, which exploit electric-field induced refractive index changes, have been realized in Si₃N₄ PICs operating in the visible spectrum by integrating thin-film materials such as ferroelectric lead zirconate titanate PZT 16 , LiNbO₃ 17 and zinc oxide (ZnO) or zinc sulfide (ZnS) thin films 18 . More recently, electric field induced charge displacement in Si₃N₄ material has been shown to induce a second-order nonlinearity, enabling a monolithic linear electro-optic effect 19 . Pockels-based modulators offer fast, power-efficient operation but introduce elevated optical losses into the waveguide.
Stress-optic modulation, in which the refractive index changes due to mechanically induced stress, has been demonstrated for Si₃N₄ PICs operating in the visible spectrum using integrated PZT thin films [20][21][22] . Hosseini et al. demonstrated such a modulator on the TriPlex platform on Si₃N₄ at 640 nm 21 and Everhardt et al., subsequently optimized the design with a dome-structured PZT layer, achieving half -wave-voltage-length product (Vπ•cm) of 16 V•cm and a voltage length optical loss product (Vπ •L•α) is 1.6 V•dB 23 . This approach offers low-power operation and moderate speed but typically requires longer active element footprints. MEMS-based modulation, which relies on mechanical deformation of the waveguide to alter the optical path length and refractive index change due to the mechanical induced stress, has been demonstrated in visible spectrum Si₃N₄ PICs using electrost
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